1. Field of the Invention
This invention relates to a synchronization signal separating a circuit for separating synchronization signal from a composite video signal.
2. Description of the Prior Art
In general, it is desirable that a synchronization signal separating circuit for separating a synchronization signal from a composite video signal have, the following characteristics:
(1) The operation of the synchronization signal separating circuit is not influenced by a curve of the front edge of the synchronization signal; PA1 (2) The operation of the circuit is not influenced by superimposed noise on the synchronization signal; PA1 (3) A large allowable limit for a level change of the composite video signal can be accomodated, and a composite video signal of poor quality with shrinking synchronization signal can be used without any trouble or mal-operation of the circuit; PA1 (4) Use of composite video signal with sag is allowable.
It is very difficult to fulfill all of these requirements, as mentioned below in more detail, and therefore the conventional synchronization signal separating circuit was designed in a compromising way.
FIG. 1 shows various composite video signals for an explanation of the required performance. An ideal waveform is shown in FIG. 1(a). FIG. 1(b), however, shows a waveform with curves at the front edges of a synchronization signal. In general, a separation of the synchronization signal from the composite video signal is made by using a preset threshold level. When a voltage V.sub.1 is set as the threshold level as shown in FIG. 1(b), it is impossible to obtain a separated synchronization signal having front edges at the position identical to that of the original synchronization signal which is the component of the composite video signal. Where a voltage V.sub.2 as shown in FIG. 1(b) is the threshold level, the synchronization signal can be accurately separated.
The curves at the front edges of the synchronization signal, such as that of FIG. 1(b), almost always appear in a composite video signal as reproduced by a simplified magnetic video recoder for home use. The genesis of such curves will be explained with reference to FIG. 2. FIG. 2(a) shows one example of an original composite video signal. FIG. 2(b) shows the composite video signal wherein the high frequency component thereof is emphasized by a pre-emphasis circuit before frequency modulation. The pre-emphasized composite video signal is clipped at higher and lower predetermined levels as shown in FIG. 2(c) in order to limit the width of side bands of the modulated signal. This is an operation necessary for the simplified magnetic video recoder for home use, since recordable bandwidth thereof is narrow. When the operated composite video signal is modulated and recorded as a FM signal and the recorded FM signal thereafter is reproduced, the composite video signal as illustrated in FIG. 2(c) can obtained. However, the waveform of the reproduced composite video signal is deformed as shown in FIG. 2(d) by a deemphasis circuit. When comparing the deformed composite video signal (FIG. 2(d)) with the original composite video signal (FIG. 2(a)), curving of the front edges of the synchronization signal is readily apparent. In recent years, with the advancement of the record density on the magnetic recording tape, the strength of the preemphasis circuit becomes stronger, and therefore, the phenomenon of such curving of the synchronization circuit is getting worse. The strength of the preemphasis is closely related to the S/N ratio, the frequency characteristics, etc. of the reproduced composite video signal. Accordingly, the strength of the preemphasis can not be easily be decreased in order to avoid such curving.
The required performance of the synchronization signal separating circuit is elucidated again referring to FIG. 1. FIG. 1(c) shows a composite video signal with noise. If the threshold level is set as a voltage V.sub.3 as shown in FIG. 1(c), accurate operation is performed. However, if the threshold level is considerably higher or lower than the voltage V.sub.3, the superimposed noise will influence the separating operation, and accurate operation can not be expected.
Next, FIG. 1(d) shows a composite video signal including synchronization signals of changing amplitudes. In FIG. 1(d), the signal level of a period T.sub.2 of the composite video signal goes down as compared with that of the period T.sub.1, and the synchronization signal of the period T.sub.3 is small as compared with that of the period T.sub.1. Such shrinking of the synchronization signal may appear when the television broadcasting wave is transmitted via many tandem offices, or by some television broadcasting wave of a CATV. If the threshold level is a voltage V.sub.4 as shown in FIG. 1(d), the separated synchronization signal becomes erroneous for the composite video signal of the periods T.sub.2 and T.sub.3. In this case, the threshold level must be a voltage V.sub.5 as shown in FIG. 1(d) in order to separate the synchronization signal accurately.
Finally, FIG. 1(e) shown an example of a waveform of a composite video signal with sag due to poor low frequency characteristics in the transmitting system. If the threshold level is set at a voltage V.sub.3 as shown in FIG. 1(e), there is a period when the synchronization signal can not be separated.
It is very difficult to fulfill all the requirements 1, 2, 3 and 4 listed above. The detecting level should be selected as high as possible in order to improve the abovementioned items (1) and (4) as shown in FIGS. 1(b) and (e), and should be selected in the middle level between that of the front or back porch (portions of the composite video signal immediately following the sync pulses) and of the sinc chip in order to improve the item (2). On the contrary, the detected level should be selected as low as possible in order to improve the item (3). Therefore, in practice, a compromise between these items is made according to some priority order. Alternatively, an automatic gain control circuit is preferably provided at the pre-stage of the synchronization signal separating circuit in order to lighten the condition of the item (3).